118 J. Thomas Patterson. 



24, B). Here, the lower portions of the two cells (on right) appear 

 to bo "buds" or outgrowths from the floor of the cavity; but if the 

 succeeding sections on each side are carefully examined, it will be 

 found that such "buds" are nothing more nor less than the lower 

 ends of cells whose upper portions reach the surface of the blasto- 

 derm in another section — and vice versa, the cells that appear to 

 form a free upper layer are found to have portions extending 

 obliquely downward to, and connecting with, the floor. 



This point is of considerable interest, because a failure to appre- 

 ciate its import probably has been the source of error on the part 

 of some embryologists (e, g., Duval), who have stated that cells are 

 cut off from the floor and are added upward to the blastoderm, thus 

 contributing to its increase in thickness. The deception arising 

 from the appearance of these buds is all the more striking in the 

 cases where the nucleus of the cell concerned lies deep enough to 

 be included in the "bud" (e. g., Fig. 24 B, n). In some sections 

 both the upper and lower portions of the cell are included (Fig. 24 

 B, c), and in such the connection of the cell with the floor is clearly 

 shown. In later stages the connection will be severed by the exten- 

 sion of the horizontal cleavage planes. 



The accessory cleavage has entirely disappeared by the thirty- 

 two celled stage, but occasionally a supernumerary sperm nucleus 

 will be found. The sections of the egg considered just above were 

 subjected to a very careful examination for the purpose of determin- 

 ing how many such nuclei were present. This study gave the fol- 

 lowing results : In the sixteen central cells eighteen nuclei were 

 found, but three of the cells had two nuclei each; that is, the cyto- 

 plasmic division of these three cells had not yet taken place. In the 

 sixteen marginal cells, together with the surrounding periblast, nine- 

 teen nuclei were found, and twp of the marginal cells had two nuclei 

 each. The extra nucleus was found far out in the periblast — so far 

 out, indeed, that there is no possibility of its being considered as a 

 derivative of a marginal cell nucleus. It must be, therefore, a sperm 

 nucleus. This could have been determined without counting the 

 other nuclei, for the one in question is in an advanced stage of 

 degeneration, and rapidly disappearing. 



It is certain, therefore, that not only does the accessory cleavage 



